图D shows examples of 数字血管造影 (DA) images acquired at the six dose settings (0.24 through 12) that are available on this Siemens angiography unit. These digital angiograms are examples of digital photo spot images, 并且通常以相对较高的剂量率获得. Data pertaining to the image acquisition techniques for the images depicted in Figures D and E are summarized in the Table below.

图D. Examples of Digital Angiograms obtained at dose levels ranging from 0.24 to 12; note that the image in the right bottom corner used approximately 50 times more radiation than the image in the upper left corner. 左上角= 0.24、中上= 0.48，右上= 1.2，左下= 2.4、中下= 4.8，右下= 12.0.

表1. 剂量设置摘要, x射线管电压, and entrance skin air kerma values corresponding to the images depicted in Figures D and E.

剂量设定	x射线管电压	入口风量(mGy)
0.24	70	0.044
0.48	70	0.083
1.2	70	0.20
2.4	70	0.39
4.8	70	1.3
12	75	2.3

The data shown in the Table indicate that the dose setting is approximately proportional to the amount of radiation that is emitted by the x-ray tube. 从0开始改变剂量设置.24 to 12 will change patient doses by approximately a factor of 50. 相应的, that it is very important for operators to understand the role of these settings, and to use these in an appropriate manner when scanning patients.

It is interesting to note that the system generally attempts to maintain an x射线管电压 of 70 kV, which will optimize the visibility of structures that contain iodinated contrast material. The reason for this is that iodine has a k-shell binding energy of 33.2 keV, and x-ray attenuation is highest just above this photon energy. Lower x射线管电压s would suffer from poorer patient penetration whereas higher x射线管电压s increase the average photon energy which reduces attenuation by iodine (absorption of x-rays by iodine is proportional to 1/E3). At the highest dose setting of 12 the voltage is increased to permit sufficient x-rays to be produced.

There are limits to increasing x-ray tube output by increasing the mA (power limitations) as well as 曝光时间 (long 曝光时间s result in increased patient motion) which are described below.

管电流(mA). The power loading of an x-ray tube is the product of the tube current (mA) and the x射线管电压 (kV), 单位是瓦特或千瓦时. 剂量设置为4时的图像.图D中的8在管电流为600 mA时得到, 负荷功率是42,000瓦.e., 42 kW). This may be compared with a typical maximum power capacity of an x-ray tube in interventional radiology of 80 or 100 kW.

曝光时间. The 曝光时间 in radiography needs to be kept short to minimize any unnecessary patient motion. 剂量设定为4.8 in 图D, the 曝光时间 was ~30 ms, which is typical of DA and DSA images.

In all of radiography, the choice of optimal techniques must take into account the detection task (e.g., 碘化造影剂的存在), x射线发生器的功率限制, 阳极散热, 曝光时间, 以及相应的病人剂量. 在这里描述的示例中, at the highest dose setting of 12 the manufacturer has elected to increase the x射线管电压 from 70 to 75 kV. 增加x射线管电压会降低图像对比度, but will also help to reduce patient dose and total 曝光时间. Radiologists need to be aware of the complex interplay of techniques on patient dose and image quality, as well as the technical capabilities of the imaging equipment that they use.

Note that the dose per frame in 数字血管造影 is approximately the same as in 数字减影血管造影 for a given dose setting. 例如, selection of the highest dose setting of 12 will result in skin doses of the order of 2 to 3 mGy in both DA and DSA imaging. 然而, 重要的是要注意在DSA成像, 血管系统单独可见, and the random noise (mottle) as depicted in Figure C is readily visible. By contrast, images in Figures D and E show that random noise is much more difficult to see. 结果是, noise (mottle) is generally less of an issue for images containing anatomy, because vessel visibility is affected by anatomical structures in the vicinity of the vasculature of interest. 因此, digital spot images are normally acquired at lower doses than those associated with DSA (see above).

图E shows enlarged images of DA images of the pelvic phantom that illustrates how vessel visibility changes with changes in radiation dose level. Changing the dose in DA images (Figures D and E) is less important than in DSA images (Figures B and C). It is therefore very important for radiologists to identify the amount of radiation that is required for a satisfactory diagnosis, and help keep patient doses As Low As Reasonably Achievable (ALARA). Application of the ALARA principle needs to be performed separately in both DA and DSA imaging, since the diagnostic tasks and image appearances are generally different. Optimal dose levels in DA imaging are should be lower than those used for DSA imaging.

图E. Regions of interests of Digital Angiograms (DA) of a pelvic phantom with added tubing containing diluted iodinated contrast medium obtained at six different dose settings, illustrating the changes of quantum mottle on overall visibility of simulated blood vessels.